A tried-and-true theory about dinosaurs may not hold up as well as scientists had once guessed.
Paleontologists who study the thermodynamics of dinosaur metabolism have suggested that, due to their immense body size, and their correspondingly small surface area to volume ratio, dinosaurs were able to keep their bodies at a relatively warm average temperature while relying on external rather than internal sources of heat. They have, almost across the board, been classified as ectothermic: cold-blooded. But this doesn’t account for the vast amounts of energy that would have been needed to power their muscles.
How could dinosaurs have managed to produce enough energy to power their gigantic muscle mass? This was the question scientists from the University of Adelaide in Australia, set out to answer in a study published July 5 in PLOS ONE on dinosaur power output. Lead author Roger Seymour measured the amount of energy exerted by crocodiles – cold-blooded reptiles that represent some of the closest living analogs to ancient dinosaurs – and compared it with that of warm-blooded mammals.
Crocodiles, the study authors found, produce much less energy in proportion to their body size than do warm-blooded, or endothermic, mammals. A 200 kg crocodile – on the large end for crocs – produces 14% of the energy that a warm-blooded mammal of the same size would.
The study found that crocodiles are lacking in endurance as well. Their energy output quickly decreases as they continue to exercise – degrading at a rate much higher than that of endothermic animals.
But the crocodiles in Seymour’s study, like dinosaurs, could still maintain a high body temperature – they just couldn’t use it to power muscles as readily. The key to this difference, Seymour believes, is in the tiny cellular components that are constantly building the molecules that provide energy for life: mitochondria. Mammals have four times as much mitochondrial surface area – where the energy-producing reactions take place – as do reptiles. Thus, mammals can produce more energy to heat their bodies and subsequently power muscles.
So what does this mean for dinosaurs? Perhaps they were not, in fact, ectothermic creatures – or at least not all of them. During their heyday in the Mesozoic, they succeeded in out-competing mammals for resources. Whether or not this was due to endothermic temperature regulation is still up in the air.
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The research reported in this news article is significant and very interesting.
However, the premise set out in the first two paragraphs above to hype the story is simply rubbish.
The statement that “[Dinosaurs] have, almost across the board, been classified as ectothermic – cold-blooded” overlooks a lively debate on the nature of temperature regulation in dinosaurs that has gone on since the mid-1960’s. An enormous amount of published evidence clearly shows that, at the very least, dinosaurs were not typically reptilian ectotherms. On this, virtually all specialists agree. Certainly, there continues to be lively debate over the the extent to which dinosaurs were heterotherms or inertial homoiotherms, as opposed to true tachymetabolic endotherms like mammals and birds. Dinosaurs may well have been metabolically unique, but in any case they were not typical “reptiles”, as Owen and Huxley already suspected in the 19th Century.
It does not serve good new science well to set its results up against an alternative that has been generally discounted for almost half a century!